The increasing use of electronic devices and electric vehicles continues to drive up the demand for ever improved power sources. Among the rechargeable power sources, alkaline rechargeable batteries are considered a family of the most promising for their desirable energy density and power. Several alkaline electrode systems have been developed for use in alkaline secondary batteries including nickel/cadmium (Ni/Cd), nickel/iron (Ni/Fe), nickel/hydrogen (Ni/H2), nickel/zinc (Ni/Zn), and nickel metal hydride (Ni/MH). Among these systems, Ni/MH batteries showed the greatest application due to their relative environmental safety and high energy density.
When an electrical potential is applied between the cathode and a suitable anode in a metal hydride alkaline cell, the negative electrode material is charged by the electrochemical absorption of a single hydrogen to form a metal hydride (MH) and the electrochemical evolution of a hydroxyl ion. For a Cd(OH)2 based anode material, as another example, two electron reactions are possible whereby the material reacts with two electrons forming metallic Cd and two hydroxyl ions. These reactions are reversed during discharge.
The reactions that take place at the positive electrode of a nickel cell are also reversible. The following charge and discharge reactions take place at a nickel hydroxide positive electrode.
Thus, an alkaline cell employing an anode material capable of multi-electron transfer should provide superior energy density.
Despite expected increases in energy density, prior cells employing anode materials capable of multi electron reactions each suffer significant drawbacks. Some of the earliest successful batteries of this type, Ni/Cd, provided insufficient energy density and suffer the drawback of severe environmental unfriendliness. Cd is a toxic heavy metal requiring specialized care during disposal. Ni/Fe batteries have been available for over 100 years and do not suffer the issues of environmental toxicity and are extremely tolerant of abuse. Ni/Fe batteries also have the advantage of being based on abundant material sources. However, Ni/Fe batteries suffer from relatively low energy density and low rate. Ni/Zn secondary batteries have been explored and used for over a century. These battery systems suffer from zinc hydroxide tending to dissolve into solution resulting in the formation of dendrites that reduce charging performance producing low cycle life.
As will be explained hereinbelow, the present invention is directed to compositionally and structurally disordered nickel based alloy materials used as an anode in a Ni/Ni cell that for the first time demonstrates the ability of Ni(OH)2 to be useful as a negative electrode material in an alkaline rechargeable cell. For a Ni(OH)2 material as provided herein used in as a negative electrode material, the half-cell reaction during charge is:Ni(OH)2+2e−→Ni+2OH−illustrating the multi electron transfer potential of this material.
These and other advantages of the invention will be apparent from the drawings, discussion, and description which follow.